Optical disc and optical disc device

ABSTRACT

In a multilayer optical disc having information layers conforming to a plurality of different optical disc standards, because the type of each information layer is not recorded in the other information layers, in read and write operations by a compatible optical disc device conforming to a plurality of optical disc standards, every time the information layer being accessed changes, it has been necessary to read the type of the information layer and select a method of generating a tracking error signal adapted to the type of information layer, so access has taken time. In order to solve the above problem, in the optical multilayer disc according to the present invention, having information layers conforming to a plurality of different optical disc standards, in an area in one of the information layers, information about the other information layers is recorded. The time required to access the other information layers can be reduced by using this information to select a tracking error signal generating method.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 14/062,152filed Oct. 24, 2013, which is a Divisional of co-pending applicationSer. No. 13/551,113, filed Jul. 17, 2012, which is a Divisional ofapplication Ser. No. 11/822,249, filed Jul. 3, 2007 (now U.S. Pat. No.8,248,907, issued Aug. 21, 2012), which is a Divisional of applicationSer. No. 11/579,832, filed on Nov. 7, 2006 (now U.S. Pat. No. 8,036,086,issued Oct. 11, 2011), for which priority is claimed under 35 U.S.C.§120. Application Ser. No. 11/579,832 is the national phase of PCTInternational Application No. PCT/JP2005/015160 filed on Aug. 19, 2005under 35 U.S.C. §371. The entire contents of each of theabove-identified applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a multilayer optical disc havinginformation layers conforming to a plurality of different optical discstandards and an optical disc device for recording or reproducinginformation on the optical disc.

BACKGROUND ART

Optical discs are a subject of research in the information recordingfield. They have a wide range of applications, from industrial use toconsumer use, because they enable non-contact recording andreproduction, because they can accommodate large files at low cost, andbecause reproduction-only, writable, or rewritable media can be selectedaccording to the application.

The capacity of optical discs has been increased by reducing the size ofthe information stored on the tracks, by using laser beams havingshorter wavelengths as light sources for recording and reproducing, byusing objective lenses with higher numerical apertures, and by reducingthe size of the focused light spot on the focal plane.

For example, in a CD (compact disc), the thickness of the disc substrateused as a light transmitting layer is about 1.2 mm, the wavelength ofthe laser beam is about 780 nm, the numerical aperture (NA) of theobjective lens is 0.45, and the disc capacity is 650 MB. In a DVD(digital versatile disc), the thickness of the disc substrate used as alight transmitting layer is about 0.6 mm, the wavelength of the laserbeam is about 650 nm, the numerical aperture of the object lens is 0.6,and the disc capacity is 4.7 GB. For a DVD, two disc substrates havingthicknesses of about 0.6 mm each are laminated together, to obtain adisc having a thickness of about 1.2 mm.

A BD (Blu-ray disc) disc having a still higher density uses an opticaldisc having a protection layer as thin as 0.1 mm as the lighttransmitting layer on the optical recording layer. With a laser beamwavelength of about 405 nm and a numerical aperture of 0.85, capacitiesin excess of 23 GB are obtained.

Thus, as optical disc capacities have increased, various optical discstandards have been established, including CD, DVD, and BD. Optical discrecording and reproducing devices are generally compatible with aplurality of different optical disc standards.

For example, some DVD recording and reproducing devices can record andreproduce information not only on a DVD but also on a CD. Some BDrecording and reproducing devices can record and reproduce informationon BDs, DVDs, and CDs. These compatible recording and reproducingdevices are extremely convenient because they allow the user to recordand reproduce information on optical discs in the user's possession thatconform to old standards, and they have played an important role insmoothing the introduction of new standards.

Although discs have been manufactured according to each of the differentstandards, to increase the user's convenience, a multilayer optical dischaving information layers conforming to a plurality of different opticaldisc standards has been developed (for example, patent document 1).

Patent document 1: Japanese Unexamined Patent Application PublicationNo. 2004-95005

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

A problem with such optical discs as the above is that when oneinformation layer is being accessed, it is not known whether anotherinformation layer is present or not, so if the need to reproduce orrecord information on another information layer suddenly arises, ittakes time to access the other designated information layer.

Another problem with optical discs such as the above is that the type ofone information layer is not recorded on another layer, so when anoptical disc device conforming to a plurality of different optical discstandards reads or writes information, every time the device accesses adifferent information layer, it must read the type of the layer toselect a method of generating a tracking error signal adapted to thetype; consequently, access takes time.

The purpose of the present invention is to shorten the access time in amultilayer optical disc having information layers conforming to aplurality of different optical disc standards.

Means of Solution of the Problems

In a multilayer optical disc having information layers conforming to aplurality of different optical disc standards, the present invention isan optical disc in which an information layer of highest recordingdensity has a management area in which information indicating the typesof other information layers is recorded, the types including at least areproduction-only type.

In a multilayer optical disc having information layers conforming to aplurality of different optical disc standards, the present invention isalso an optical disc in which an information layer of highest recordingdensity has a management area in which information indicating presenceor absence of other information layers is recorded.

Effect of the Invention

In a multilayer optical disc having information layers conforming to aplurality of different optical disc standards, the present inventionenables the access time to be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an optical disc and optical disc devicein a first embodiment, showing the optical disc device accessinginformation layer L1 of the optical disc.

FIG. 2 is a schematic drawing of the optical disc and optical discdevice in the first embodiment, showing the optical disc deviceaccessing information layer L2 of the optical disc.

FIG. 3 is a schematic drawing of the optical disc and optical discdevice in the first embodiment, showing the optical disc deviceaccessing information layer L3 of the optical disc.

FIG. 4 is a schematic drawing showing the management area in informationlayer L1 of the optical disc in the first embodiment.

FIG. 5 is a schematic drawing of the data structure of the optical discin the first embodiment, showing an information field in the managementarea.

FIG. 6 is a conceptual drawing of the information field in themanagement area in the first embodiment.

FIG. 7 is a schematic drawing showing a focused spot and informationwritten in tracks on information layer L2 in the first embodiment.

FIG. 8 is a schematic drawing showing a focused spot and informationwritten in tracks on information layer L3 in the first embodiment.

FIG. 9 is a flowchart illustrating a sequence that reads informationabout the individual types of information layers in the firstembodiment.

FIG. 10 is a flowchart illustrating a sequence that uses the informationabout the individual types of information layers in the first embodimentto reproduce information from another information layer.

FIG. 11 is a schematic diagram showing the management area ininformation layer L2 in a second embodiment.

FIG. 12 is a conceptual drawing of parts of the management area in thesecond embodiment.

FIG. 13 is a schematic diagram showing parts of a management area in athird embodiment.

EXPLANATION OF REFERENCE CHARACTERS

1 optical disc, 2 a blue-violet semiconductor laser light source, 2 bred semiconductor laser light source, 2 c infrared semiconductor laserlight source, 3 a beam from blue-violet semiconductor laser lightsource, 3 b beam from red semiconductor laser light source, 3 c beamfrom infrared semiconductor laser light source, 4 a, 4 b, 4 ccollimating lens, 5 a, 5 b, 5 c prism, 6 objective lens, 7 a focusedspot from blue-violet semiconductor laser light source, 7 b focused spotfrom red semiconductor laser light source, 7 c focused spot frominfrared semiconductor laser light source, 8 sensor lens, 9photodetector, 10 signal processing means, 11 image processing means, 12optical disc device, 13 host device, 14, 14 a management area, 15, 15 auser data area, 16, information field, 17 land, 18 groove, 19 recordingmark, 20 pit, 21 indicator area, 22 type area

BEST MODE OF PRACTICING THE INVENTION First Embodiment

FIGS. 1 to 3 are schematic drawings showing an optical disc and anoptical disc device accessing an information layer in the optical discin the first embodiment of the invention; FIG. 4 is a schematic drawingshowing the management area in information layer L1 of the optical discin FIG. 1. FIG. 5 is a schematic drawing showing the data structure ofthe optical disc, including the information field in the managementarea. FIG. 6 is a conceptual drawing of the information field in themanagement area. FIG. 7 is a schematic drawing showing a focused spotand information written in tracks on information layer L2; FIG. 8 is aschematic drawing showing the focused spot and information written intracks on information layer L3. FIG. 9 is a flowchart illustrating asequence that reads information about the individual types ofinformation layers. FIG. 10 is a flowchart illustrating a sequence thatuses the information about the individual types of information layers toreproduce information from another information layer.

The first embodiment of the invention will now be described withreference to the attached drawings. The optical disc 1 in the firstembodiment is provided with information layers L1, L2, and L3 for aplurality of differing optical disc standards. Information layers L1,L2, and L3 are disposed at distances of 0.1 mm, 0.6 mm, and 1.2 mm fromthe surface, in this order. These information layers L1, L2, and L3satisfy the BD, DVD, and CD optical disc standards, in this order,information layer L1 being the layer with the highest recording density,information layer L3 being the layer with the lowest recording density.In the following description, these layers are a reproduction-onlylayer, a writable layer, and a reproduction-only layer, respectively.

In the state in FIG. 1, information layer L1 is being accessed; a beam 3a emitted from a blue-violet semiconductor laser light source 2 a, whichhas a wavelength of about 405 nm, this being the shortest wavelength, iscollimated by a collimator lens 4 a, reflected by a prism 5 a, andfocused to a focused spot 7 a on information layer L1 by an objectivelens 6.

The beam 3 a reflected from information layer L1 returns to theobjective lens 6, then passes through prisms 5 a, 5 b, 5 c and isdirected by a sensor lens 8 onto a photodetector 9.

The photodetector 9 converts the received light beam into electricalsignals for output to a signal processing means 10. The signalprocessing means 10 controls the amount of light emitted by light source2 a, generates a tracking error signal indicating the relativepositional discrepancy between the focused spot and the tracks on theinformation layer, and after signal processing, outputs information toan image processing means 11. The image processing means 11 performsimage processing and output.

The above elements from the laser light source 2 a to the imageprocessing means 11 constitute an optical disc device 12. The opticaldisc device 12 records and reproduces information on the optical disc 1according to commands from a host device 13. The host device 13 is a PC(personal computer) that instructs the optical disc device 12 to recordand reproduce information. Alternatively, the host device 13 may be anaudiovisual reproduction device that decodes information read out fromthe optical disc device 12 and outputs images and/or sounds, and/or anaudiovisual recording device that encodes image and/or sound informationinput from an external device and instructs the optical disc device 12to record the information.

In the state in FIG. 2, information layer L2 is being accessed. A beam 3b having a wavelength of about 650 nm is emitted from a redsemiconductor laser light source 2 b, collimated by a collimator lens 4a, reflected by prism 5 b, and focused to a focused spot 7 b oninformation layer L2 by the objective lens 6.

The beam 3 b now passes through information layer L1. The material andstructure of information layer L1 are selected so as to avoidinterference with access to information layer L2, taking physicalproperties such as transmittance and reflectance of a light beam havinga wavelength of about 650 nm into consideration.

The beam 3 b reflected from information layer L2 returns to theobjective lens 6, then passes through prisms 5 a, 5 b, 5 c and isdirected by the sensor lens 8 onto the photodetector 9. The signalprocessing means 10 and image processing means 11 operate as above;descriptions will be omitted. In FIG. 2, the signal processing means 10controls light source 2 b.

In the state in FIG. 3, information layer L3 is being accessed. A beam 3c emitted from an infrared semiconductor laser light source 2 c having awavelength of about 780 nm is collimated by a collimator lens 4 c,reflected by a prism 5 c, and focused to a focused spot 7 c oninformation layer L3 by the objective lens 6.

The beam 3 c now passes through information layers L1 and L2. Thematerials and structure of information layers L1 and L2 are selected soas to avoid interference with access to information layer L3, takingphysical properties such as transmittance and reflectance of a lightbeam having a wavelength of about 780 nm into consideration.

The beam 3 b reflected from information layer L2 returns to theobjective lens 6, then passes through prisms 5 a, 5 b, 5 c and isdirected by the sensor lens 8 onto the photodetector 9. The signalprocessing means 10 and image processing means 11 operate as above;descriptions will be omitted. In FIG. 3, the signal processing means 10controls light source 2 c.

The management area 14 disposed in information layer L1 in FIG. 4 andFIG. 5 is located at the innermost circumference of the optical disc 1,the rest of the area being a user data area 15. The information field 16in this management area 14 contains information indicating the type ofeach information layer. The optical disc device 12 obtains informationindicating the types of information layers by reading the informationfrom the information field 16.

FIG. 6 is a conceptual drawing of the information field 16. Areas S1,S2, S3 in the information field 16 are bit areas indicating the types ofinformation layers L1, L2, L3, respectively. Each of areas S1, S2, S3comprises two bits, making six bits in total. When an information layeris of the reproduction-only type, the bit values are ‘01’, and when alayer is of the writable type, the bit values are ‘10’. Absence of aninformation layer is represented by bit values of ‘00’. Therefore, inthe first embodiment, areas S1, S2, S3 have values ‘01’, ‘10’, ‘01’,respectively.

FIG. 7 shows focused spot 7 b and information written on a track of awritable information layer. A writable information layer has tracksreferred to as lands 17 and grooves 18. Information is recorded asrecording marks 19 by inducing a chemical change or shape change in thematerial. In a writable information layer, a tracking error signal isgenerated by the Differential Push-Pull method (DPP method), which isknown in the art. In this method, the light is split into three beamsand focused on the information layer, the reflected light is received bythree binary photodetectors, and calculations are carried out togenerate a tracking error signal.

FIG. 8 shows focused spot 7 c and information written on a track of areproduction-only information layer. On a reproduction-only layer,information is recorded in the form of microscopic holes called pits 20.The tracking error signal is generated by the Differential PhaseDetection method (DPD method), which is also known in the art. In thismethod, the light beam is focused on an information layer, the reflectedlight of the focused light is received by one four-quadrantphotodetector, and calculations are carried out to generate the trackingerror signal.

In FIG. 9, when the optical disc 1 is inserted into the optical discdevice 12 (ST1), the optical disc device 12 reads information from theinformation field 16 in the management area 14 on information layer L1of the optical disc 1 to obtain information indicating the types ofinformation layers L1, L2, L3. The information is retained in the signalprocessing means 10 (ST3) and the process ends (ST4). The device returnsto a standby mode to wait for a command from the host device 13.

In FIG. 10, in response to a command from the host device 13 toreproduce information on information layer L2 (ST10), the optical discdevice 12 follows the sequence by reading the type of information layerL2 retained in the signal processing means 10 as above (ST11). Asinformation layer L2 is not a reproduction-only layer (FALSE in ST12)but a writable layer (TRUE in ST13), the DPP method is selected, whichis suitable for writable information layer L2, the laser power of thelaser having a wavelength suitable for information layer L2 is set atthe same time, a tracking error signal is generated (ST14), andinformation is reproduced from information layer L2 (ST15).

Similarly, in response to a command from the host device 13 to reproduceinformation from information layer L3 (ST10), the optical disc device 12follows the sequence by reading the type of information layer L3retained in the signal processing means 10 as above (ST11). Asinformation layer L3 is a reproduction-only layer (TRUE in ST12), theDPD method is selected, which is suitable for reproduction-onlyinformation layer L3, the laser power of the laser having a wavelengthsuitable for information layer L3 is set at the same time, a trackingerror signal is generated (ST16), and information is reproduced frominformation layer L3 (ST17).

When the optical disc device 12 receives the information reproducingcommand from the host device 13, if the type of information layer L3retained in the signal processing means 10 is neither thereproduction-only type nor the writable type (FALSE in ST12 and ST13),that is, if the type information is not recognizable by the optical discdevice, a notification is returned to the host device 13 indicating thatthe information layer to be reproduced is not accessible (ST18), and theprocess ends (ST19). The optical disc device 12 returns to a standbymode to wait for the next command from the host device 13.

In FIG. 10, steps ST12 and ST13 can be performed in either order todetermine the type of the information layer, that is, to determine wherethe layer is of the reproduction-only type or writable type.

FIG. 10 shows a case in which the host device 13 commands thereproduction of the information. When the host device 13 commands therecording of the information, access time can be similarly shortened andappropriate recording operations can be carried out on the informationlayer to be recorded, based on the information indicating the type ofinformation layer.

In this embodiment, in a multilayer optical disc having informationlayers conforming to a plurality of different optical disc standards,information indicating the type of other information layers is recorded,so the optical disc device can easily identify the types of the otherinformation layers, and access time to those layers can be shortened.

Furthermore, the information indicating the types of the otherinformation layers is recorded in the management area of the informationlayer having the highest recording density. It can be anticipated thatthe information layer having the highest recording density will beaccessed most frequently, so the optical disc device will be sure toread and retain the information.

When recording and reproducing information on an optical disc as above,before access, the optical disc device can set a suitable value for thelaser power of the laser having a wavelength suitable for the layer tobe accessed, based on the information indicating the type of theinformation layer. Also, the method of generating a tracking errorindicating a relative positional discrepancy between the focused spotand the information written in tracks in a direction crossing theplurality of tracks can be determined in advance, so access time can beshortened.

Second Embodiment

The second embodiment is similar to the first embodiment in regard toFIG. 1 to FIG. 5 and FIG. 7 to FIG. 10, but differs from the firstembodiment in regard to the usage of the part of the management area inFIG. 6.

Referring to FIG. 11, the optical disc comprises a management area 14 alocated at its inner circumference and a user data area 15 a extendingtoward its outer circumference. Inside the management area 14 a, anindicator area 21 is provided to record information indicating thepresence or absence of other information layers, and a type area 22 isprovided to record information indicating the types of the otherinformation layers. In FIG. 12, areas T2 and T3 of indicator area 21respectively record information indicating the presence or absence ofinformation layers L2 and L3 shown in FIG. 1. Area T4 indicates thepresence or absence of another information layer (not shown in opticaldisc 1) conforming to an optical disc standard other than the BDstandard which uses a blue-violet semiconductor laser having about a405-nm wavelength as a light source, or to a standard that may bedeveloped in the future. Areas T2, T3, T4 store one bit of informationeach, so the indicator area 21 stores three bits in total. A bit valueof ‘1’ indicates the presence of another information layer, while a bitvalue of ‘0’ indicates the absence of the other information layer.

In FIG. 12, areas U2, U3, U4 in the type area 22 are associated with theinformation represented by areas T2, T3, T4 of indicator area 21,respectively; areas U2, U3, U4 record information indicating the type ofthe corresponding information layers. That is, when areas T2, T3, T4indicate the presence of an information layer (bit value ‘1’), areas U2,U3, U4 indicate the type of the information layer. Areas U2, U3, U4contain two bits of information each, so type area 22 has six bits intotal. When a layer is of the reproduction-only type, the correspondingbit values are ‘00’. When the layer is of the writable type, the bitvalues are ‘01’, and when it is of the rewritable type, the bit valuesare ‘10’. The writable type includes the DVD−R and DVD+R standards andtheir dual-layer standards, which permit writing but not rewriting. Therewritable type includes the dual-layer DVD−RW and DVD+RW standards andDVD-RAM. When one of areas T2, T3, T4 indicates the absence of acorresponding information layer (bit value ‘0’), the value ‘00’,indicating a ‘reserved area’, is assigned to the corresponding U2, U3,or U4 area. Although, this value is apparently the same as for aninformation layer of the reproduction-only type, no particular problemoccurs, because when one of the T2, T3 and T4 bits is ‘0’, the opticaldisc device will not read the corresponding U2, U3, or U4 area.Alternatively, a bit value (for example ‘11’) other than the above canbe assigned.

When an optical disc is inserted into the optical disc device, thedevice reads the information in areas T2, T3, T4 of the indicator area21 in the management area 14 a on information layer L1 of the opticaldisc to obtain information indicating the presence or absence ofinformation layers L2 and L3 and information layers conforming to otherstandards. When one of areas T2, T3, T4 indicates the presence of acorresponding information layer (bit value ‘1’), the optical disc devicereads the corresponding one of areas U2, U3, U4 in the type area 22 inthe management area 14 a, obtains information indicating the type ofinformation layer, and retains the obtained information in the signalprocessing means 10. Subsequent operations are similar to those shown inthe flowchart in FIG. 10.

The optical disc device can be configured to read only those of theareas U2, U3, U4 in the type area 22 corresponding to areas T2, T3, T4indicating the presence of an information layer (bit value ‘1’).Alternatively, the optical disc device can be configured to readinformation in all areas U2, U3, U4 of the type area 22 regardless ofwhether the corresponding information layer is present or not.

When an information layer corresponding to area T4 is present, and theinformation layer conforms to an optical disc standard, other than theBD standard, that uses a blue-violet semiconductor laser having awavelength of about 405 nm as a light source, the disc will have aplurality of information layers (two layers, information layer L1 andthe information layer corresponding to area T4) using the shortestwavelength of approximately 405 nm; in this case, the indicator area 21and type area 22 are provided at least in information layer L1. Theindicator area 21 and type area 22 may be provided not just oninformation layer L1 but also on the information layer corresponding toarea T4.

Alternatively, areas T4 and U4 may be left empty, with no particularinformation stored, reserved for use in case a new standard is developedin the future. The indicator area 21 and type area 22 then holdinformation for two types of information layers, these being informationlayers L2 and L3.

Alternatively, the two bits assigned to area U2 may classify thecorresponding DVD layer according to its type of information trackstructure. The classification may be based on the periodic structure ofthe information tracks, e.g., on their wobble frequency. For example,when the layer is a reproduction-only type layer such as a DVD-ROM layerhaving only pre-pits with no wobble, the bit value may be ‘00’. ForDVD-R and DVD-RW, which have wobble, the bit value may be ‘01’. ForDVD+R and DVD+RW, which have wobble with higher wobble frequency thanDVD-R and DVD-RW, the bit value may be ‘10’. For DVD-RAM in which thehead is shifted in the radius direction on a land/groove single spiralstructure with no wobble, the bit value may be ‘11’. The frequencystructure of the information tracks in a DVD layer has the same formatfor each category, so the characteristics of the wobble signal detectioncircuit (a circuit such as a band-pass filter for detecting thefrequency of the periodic structure) can be set prior to access to theDVD layer, the disc rotation can be rapidly brought to the target value,and the access time can therefore be shortened.

In this embodiment, in a multilayer optical disc having informationlayers conforming to a plurality of different optical disc standards,information indicating the presence of other information layers isrecorded, so the optical disc device can easily identify the presence orabsence of other information layers, and access time can be shortened.

Furthermore, in the management region of the optical disc, a type area22 which stores information indicating the types of information layersis provided in addition to the indicator area 21 which storesinformation indicating the presence or absence of other informationlayers, and the information in the indicator area 21 is related to theinformation in the type area 22 so that the information in the indicatorarea 21 and the information in the type area 22 have a hierarchicalstructure. Therefore, if another area is created in the user data area15 in the future, or if another type of information layer is added, thehierarchical structure can be exploited to accommodate the expansion.

Furthermore, the information is recorded in the management area in aninformation layer having the highest recording density, which isexpected to be the information layer most frequently accessed, ensuringthat the optical disc device will read and retain the information.

When recording and reproducing information on an optical disc of thistype, the optical disc device can set a suitable value for the laserpower of a laser having a wavelength suitable for the layer to beaccessed, based on information such as the above. The method ofgenerating the tracking error signal indicating a relative positionaldiscrepancy between the focused spot and the information written in thetracks in a direction crossing a plurality of tracks can be determinedin advance, so the access time can be shortened.

Third Embodiment

In the third embodiment, the optical disc 1 has at most two types ofinformation layers, including the type of the L1 layer. The thirdembodiment is similar to the second embodiment in regard to FIG. 1 toFIG. 5 and FIG. 7 to FIG. 10, but differs from the second embodiment inregard to the usage of the indicator area 21 and type area 22 in FIG.12.

In FIG. 13, the areas T2, T3, T4 in the indicator area 21 are the sameas in FIG. 12.

When an information layer other than information layer L1 is present,area V1 in the type area 22 indicates the type of this informationlayer. The type area 22 includes only the two bits of information inarea V1. The values of these bits are ‘00’ when the information layer isof the reproduction-only type, ‘01’ when it is of the writable type, and‘10’ when it is of the rewritable type. The writable type includesDVD−R, DVD+R, and their dual-layer standards. The rewritable typeincludes dual-layer DVD−RW and DVD+RW and DVD-RAM.

For example when information layer L1 is a writable layer or pair oflayers, information layers L2 and L3 are reproduction-only layers, andno other layers are present (T2=1, T3=1, T4=0), the bit values in areaV1 will be ‘00’.

Thus, in the optical disc 1, when at least one of areas T2, T3, T4 inthe indicator area 21 has a bit value of ‘1’ and the correspondinginformation layers are of only one type, or when only one of areas T2,T3, T4 has a bit value of ‘1’, the type area 22 can have only two bits,producing a saving of information bits as compared with the secondembodiment above. In particular, there is an anticipated market demandfor optical discs having only one of information layers L2 and L3 andusing it as a reproduction-only layer, so if only optical discs havinginformation layer L2 or L3 and using it as a reproduction-only layer aremanufactured and sold, the two bits of area V1 will suffice for the typearea 22.

Alternatively, as shown in the second embodiment, the classification inarea V1 may be based on the type of information track structure, such asthe presence or absence of wobble.

Although the area V1 of the type area 22 was shown as having two bits ofinformation in this embodiment, it may have only one bit of information,saving one further information bit. In this case, the reproduction-onlytype may be indicated by ‘0’ and other types, that is, the writable typeand rewritable type, may be indicated by ‘1’. Alternatively, theclassification may be based on the type of information track structure:for example, ‘0’ when there is no wobble and ‘1’ when wobble is present.

Furthermore, when the area V1 storing information indicating the type ofinformation layer has only one bit and area T4 in the indicator area 21in FIG. 13 is a reserved area with no particular information, the onebit of area V1 can be reassigned to area T4, since area T4 is a one-bitarea to begin with. This scheme saves a still further information bitand, by placing the indicator area 21 and type area 22 adjacent to eachother, can further shorten the access time of the disc device.

Other effects are similar to those shown in first and secondembodiments.

Fourth Embodiment

It is possible to use the type area 22 of the second and thirdembodiments as a reserved area having a value of ‘00’ or ‘0’ with noinformation assigned, in expectation that effective use of this area canbe made in the future. Also, as described in the third embodiment, ifonly optical discs having information layers L2 and L3 of thereproduction-only type are manufactured and sold, there is no particularneed to assign information indicating the type of the information layerto the type area 22.

However, if the market changes in the future and not only optical discshaving information layers L2 and L3 of the reproduction-only type butvarious other types of hybrid optical discs are manufactured and sold,it will be necessary to use the bit or bits in the type area 22 toindicate the type of information layer. In order to ensure compatibilitywith optical discs having information layers L2 and L3 of thereproduction-only type, the bit values ‘00’ or ‘0’ should be assigned toindicate the reproduction-only type, which is the type most frequentlyused. If these bits are used for indicating some completely differentproperty, the value ‘00’ or ‘0’ should be protected in some way, such asbeing used only as a reserved value.

This embodiment provides only the information stored in the indicatorarea to indicate the presence or absence of other information layers,but since this information enables the optical disc device to identifythe presence or absence of other information layers, it contributes tothe shortening of the access time.

What is claimed is:
 1. An optical data storage medium comprising: atleast one recording layer, satisfying BD optical standard, having atrack upon which a laser beam is focused during use; and a managementarea and a user data area provided on said at least one recording layersatisfying BD optical disc standard, said management area havingrecorded data representing if another type of recording layer is presentin said medium, and said management area being located interiorly tosaid user data area, wherein said recorded data represents if saidanother type of recording layer is present at a predefined depth of saidmedium, and said recorded data represents a depth at which said anothertype of recording layer should be present, and in case that said anothertype of recording layer is not present in said medium said recorded dataindicates that said another type of recording layer is not present atsaid predefined depth, said recorded data is provided only on themanagement area of the layer satisfying BD optical disc standard, saidrecorded data represents absence of a recording layer by bit values of00.
 2. An optical data storage system comprising: a laser light sourcefor producing a laser beam; an optical data storage medium according toclaim 1; a focusing device for focusing said laser beam on the layersatisfying the BD optical disc standard of said medium; and a photodetector for detecting a return laser beam from said medium andgenerating a data signal responsive thereto, wherein said focusingdevice focuses said laser beam onto said management area to readinformation stored therein.